How do I setup my drop-down window to select the listItem?

Then we have to create our own scroll method.

This might work for you, give it a try. Pass the objects - dropdown and listItem to be selected to this method. You may also have to name map your vScroll bar accordigly else update the code as per your need.

function scrollIntoView(dropdown, listItem) {

  while ( (dropdown.Height < (listItem.Top + listItem.Height)) && 
                  ((dropdown.vScroll.Pos + 5) < dropdown.vScroll.Max)) {
    dropdown.vScroll.Pos += 5;
    Delay(100);
  }
  
  while (listItem.Top < 0 && (dropdown.vScroll.Pos - 5) > 0) {
    dropdown.vScroll.Pos -= 5;
    Delay(100);
  }

}